OP THE FAMILY OP NAIADES. 411 



"horns" more depressed and more frequent, and thus pass 

 into varieties with a mere furrow without any distinct eleva- 

 tions, and these gradations are ahnost innumerable. The 

 irroratus has slightly elevated tubercles along both sides of 

 the furrow ; these are sometimes continued along the wrin- 

 kles, making them elevated. The sesopus has a "nodu- 

 lous ridge" over the middle of the shell, and the plkatus 

 has folds or waves over the posterior part of the disks, more 

 or less numerous, and which are so large as to produce an 

 irregular effect through the nacre in many instances. 



The epidermal colours of this family are exceedingly cir- 

 cumscribed. The ground varies from deep fuscous or black 

 to pale yellow, frequently passing through obscure green, 

 rarely bright green. This ground is intersected frequently 

 with rays or spots of a darker hue. In fine and perfect spe- 

 cimens these are generally perceptible, sometimes eminently 

 beautiful. In imperfect or old specimens these marks are 

 almost always obliterated. The following species, when the 

 specimens are perfect and fresh, occur beautifully painted 

 with rays more or less broad : viz. complanatus, cuneatus, ra- 

 diatus, siliquoideus, ventricosus, ovatus*, cariosus, nasutus, 

 lacrymosus (very slightly), calceolus, rectus, ochraceus, hetero- 

 don, sulcatus, multiradiatus, occidens, iris, and zig-zag. 



The securis is rayed in a manner peculiar to itself. (See 

 description.) The cornutus has beautiful hairlike lines, some- 

 times minutely waved, which diverge to its entire margin. 

 Some of the varieties have no rays, while others have compa- 

 ratively broad and beautiful ones. The sulcatus is indistinctly 

 rayed over the umbonesf, but the furrow passing from the 

 beaks to the posterior basal margin has many hair like lines, 



* Mr Say says this shell is " not radiated." This is generally the case : but 

 some specimens are beautifully rayed ; and Lamarck says of his var. 6, " testa ra- 

 diis longitudinalibus picta." 



t I use this term as Linnasus did : it is the '■'■ventre''' of the French writers. 

 Draparnaud says, " la portion la plus renflee des valves." It is improperly used 

 by English writers denoting the beaks or summits. 



VOL. III. 5 M 



